Over the years, a wide variety of networks have been developed to carry various types of information. Early networks were telephone networks designed with voice communications in mind. These networks were, and still are, primarily circuit-based networks. In a circuit-based network, each call establishes a dedicated, point-to-point connection through the network which, for instance, allows people at both ends of a telephone call to speak and listen at the same time.
A circuit remains open for the entire duration of a call even if no one is speaking. In which case, a large portion of circuit's bandwidth, or capacity to carry information, is wasted on silence, or meaningless data. Time Division Multiplexing (TDM) is a common circuit-based technology. In TDM, analog signals are digitally coded and multiplexed in time over circuits at a constant bit rate.
In recent decades, the wide spread use of computers has led to the development of additional types of networks. These networks have been designed with data communications in mind and are primarily packet-based networks. In a packet-based network, a call may consist of a stream of data sent from one computer to another. The stream of data is divided up into packets before it enters the network. At the destination, the stream of data is re-assembled from the packets.
A packet-based call does not require a dedicated connection through the network. Instead, packets from many different calls can share the same bandwidth. That is, packets from one call can be inserted into spaces between packets from other calls. In which case, packet-based networks efficiently utilize much more network bandwidth than circuit-based networks, making packet-based networks particularly suited to handle the large volumes of data traffic.
Packet-based networks, however, normally do not work well for time critical transmissions such as voice. For instance, in packet-based networks, packets may experience delay variations while traveling through the network. As a result, packets are rarely received at a constant bit rate. In data communications, delay variations between packets usually do not matter. A computer can just wait for a complete set of packets to arrive before processing the data. For time critical transmissions however, delay variations can have a significant impact on the quality of the call. In which case, circuit-based networks like TDM are generally better suited for constant bit rate, time critical transmissions such as voice.
Since packet-based and circuit-based networks are suited to different kinds of data, network carriers often have to maintain more than one kind of network to satisfy client needs. A carrier may need to maintain TDM for voice and/or video, as well as packet-based networks such as frame relay, ATM (asynchronous transfer mode), and IP (internet protocol) for data. In order to reduce the number of networks that must be supported, a network solution is needed that can provide the advantages of both a circuit-based, constant bit rate service and a packet-based, high bandwidth utilization service.
One approach offered by an industry cooperation group, The ATM Forum, is CES (circuit emulation service) over ATM. CES over ATM is described in “Circuit Emulation Service Interoperability Specification,” AF-SAA-0032.000, published September 1995, and “Circuit Emulation Service Interoperability Specification Version 2.0,” AF-VTOA-0078.000, published January 1997, both available from The ATM Forum at http://www.atmforum.com. CES over ATM establishes a logical path through the ATM network. In this respect, CES over ATM is similar to TDM in that all the data in a circuit follows the same point-to-point path. With a common path, there should be no out-of-order packets.
An ATM path can accommodate multiple circuits. Depending on a data rate needed for a given circuit, different amounts of bandwidth can be assigned to different circuits in a path. As a result, delay variations between packets should be greatly reduced.
Theoretically, CES over ATM eliminates the need for multiple networks because it allows ATM to handle regular data as well as constant bit rate data. ATM, however, has a number of disadvantages and limitations. For instance, ATM is not as widely spread as some other networks. The smaller ATM market share has lead to less research and development directed to future improvements, gaps in ATM availability, especially between regions serviced by different network carriers, and more expensive ATM hardware and technical support. Other limitations include security, in that ATM packet headers cannot be encrypted, and failure recovery, in that data is often lost and re-routing is slow compared to some other networks. For these and numerous additional reasons, CES over ATM is less than an ideal network solution for constant bit rate data transmission.